What influences the visibility of the layer selection in the view port header?
Local and Global View

From the Blender wiki:  


  You can toggle between Local and Global view by selecting the option
  from the View Menu or using the shortcut NumPad /. Local view isolates
  the selected object or objects, so that they are the only ones visible
  in the viewport. This is useful for working on objects that are
  obscured by other ones, or have heavy geometry. Press NumPad / to
  return to Global View.


As user2859 pointed out, when you are in Local View, the Layers buttons are not visible, but will return when you go back to Global view.